Dietary fiber shows various physiological actions including improvement in intestinal environment, suppression of elevation of blood sugar level, and suppression of cholesterol level, intake of which is however believed to be insufficient in Japan, Europe and the United States. While dietary fiber may be taken from various plants, it has to be highly purified before being applied to a wide variety of processed foods, so that the cost will elevate due to complicated processes necessary therefor. Use of dietary fiber as a partial substituent of cereal flour raises a problem that the dietary fiber has physical properties different from those of starch component which is a main constituent of cereal flour, and therefore largely affects taste and processes.
On the other hand, starch is readily digestible in general, but contains an indigestible fraction called resistant starch. It has been becoming clear that the resistant starch acts similarly to dietary fiber in vivo. Reported nutritional benefits include improvement in intestinal environment, suppression of elevation of blood sugar level, suppression of cholesterol level, and improvement in lipid metabolism.
Starch is abundant in plants, and is relatively easy to purify. Starch may, therefore, be supplied at lower cost than dietary fiber. Moreover, since resistant starch-containing starch is partially substitutable for cereal flour such as wheat flour, so that it is miscible in a relatively easy manner, without heavily affecting the original processes or ingredient. However, there has been a practical limit in the ratio of substitution by the resistant starch-containing starch. Resistant starch content in intact resistant starch-containing starch has been found to be 45% or less in general. A problem has, therefore, remained in that resistant starch content cannot be elevated as expected from the amount of addition, even if it were added to foods.
In this situation, there has been reported techniques of producing processed starches which are enriched in resistant starch by processing raw starch.
Patent Document 1 (International Publication WO2000/19841, pamphlet) describes acid treatment of high-amylose corn starch, used as a raw material, in alcohol. The processed starch obtained by the acid treatment, having the number-average molecular weight in the range from 10,000 to 90,000, reportedly expresses slow digestivity in vivo.
Patent Document 2 (Japanese Laid-Open Patent Publication No. 2001-231469) describes that high-amylose starch, used as a raw material, is heated under a water percentage and temperature insufficient for decomposing granularity of starch, and is digested to remove the amorphous portion, to thereby enhance the resistivity. More specifically, HYLON VII (registered trademark, a corn starch with 70% amylose content), used as a raw material, was heated under a total water percentage of approximately 38%, at approximately 98.9° C. for 2 hours, and digested with pancreatin. The thus-obtained starch reportedly had a total dietary fiber (TDF) of 50%, and a resistant starch (RS) content of 90% (Example 1a). The document also describes that the thus-obtained resistant starch was found to show a molecular weight peak over the range from 2,000 to 80,000, and a heat of gelatinization of approximately 20 J/g.
Patent Document 3 (Japanese Laid-Open Patent Publication No. H11-5802) describes a technique of enriching hardly-digestible component, by keeping an aqueous dispersion of high-amylose starch at a temperature not lower than the temperature at which the starch components starts to elute, not higher than the temperature at which the starch begin to decompose its granularity, and by allowing α-amylase to react therewith. The document describes an exemplary case where the hardly-digestible component was enriched up to 68.2% when measured according to the Prosky method (Example 3).
Patent Document 4 (Published Japanese Translation of PCT International Publication No. 2008-516050) describes that production of an enzyme-resistant starch by using high-amylose starch as a raw material, and heating it in a moist state under the presence of alcohol. The document describes an exemplary case where the total dietary fiber content was reached approximately 60 to 70%, and the Englyst resistant starch value was averaged at 43 (Example 4).
Patent Document 5 (Japanese Laid-Open Patent Publication No. H10-195104) discloses a processed starch enriched with dietary fiber, obtained by subjecting high-amylose corn starch to heat-moisture treatment.
Patent Document 6 (Japanese Laid-Open Patent Publication No. H09-12601) discloses a method of producing amylase-resistant starch, obtained by subjecting high-amylose starch to heat-moisture treatment.
Patent Document 7 (Japanese Laid-Open Patent Publication No. H10-191931) discloses a resistant starch obtained by subjecting starch to debranching and retrogradation treatment.
Patent Document 8 (Published Japanese Translation of PCT International Publication No. H05-506564) describes a technique of enzymatic hydrolysis of retrograded high-amylose starch.
Patent Document 9 (International Publication WO2008/155892, pamphlet) describes a method of producing a resistant starch-rich starch, by bringing high-amylose corn starch into contact with pressurized hot water at 165° C. to 260° C.
Patent Document 10 (Japanese Laid-Open Patent Publication No. 2008-280466) describes a technique of producing amylose grains making use of an enzyme reaction. According to the description, the amylose grains obtained by the method of this document has a specific weight-average molecular weight and molecular weight dispersion, and is substantially resistant against digestion by α-amylase.